Griefing

From TF2 Wiki

Griefing is the act of using or abusing a game mechanic in ways originally unintended by the game's developers for the specific purpose of angering or otherwise negatively impacting the game experience of another player. Griefing can sometimes be difficult to pinpoint because its defining characteristic is intent and not action.

Griefing is distinctly different than "being an idiot." Bug abuse and hacking can be griefing, but only when done specifically to disrupt another player's gameplay, and not when done simply to gain an unfair advantage.